Properly release internal data on error - take 2

This pull-request replaces the original solution from !7 (merged), after calling adcli_conn_unref() worked in one place I copied it to various other places without checking if it really fits there. This caused other issued since some expected data was not available anymore.

The new solution tries to avoid a direct termination of adcli by replacing errx() with warnx() so that the current adcli task can return properly to main() where adcli_conn_unref() is called.